This invention relates to the production and use of nanostructured layered lithium manganese oxide powders (i.e., primary particles of less than 300 nm and preferably in the size range of 5–100 nm) as active cathode materials in Li-ion and Li rechargeable batteries. A layered manganese oxide is a rock salt-structured material based on a close packed network of oxygen atoms with Li+ and Mn3+ ions ordering on alternating (111) planes of the cubic rock salt structure. Host electrodes with layered structures are of particular interest to the battery industry because they provide a two-dimensional interstitial space that allows for rapid lithium ion transport within the host (see M. M. Thackeray, Prog. Solid. St. Chem., 25, 1 (1997)). The electrochemical performance of nanostructured materials can exceed that of large-sized (coarse) particles of the same materials, because of the better diffusion of Li ions in nanostructured electrode materials during the discharging process and the smaller dimensional changes in cathode materials as Li ions cycle in and out during the discharging and charging process.
Rechargeable batteries are being used in increasing number in several military and commercial applications. They are also used to power portable equipment (e.g., power tools, camcorders and wireless communication devices). There is a tremendous demand for reliable and safe rechargeable batteries with high energy density, good cycling life, light-weight, and low cost. Rechargeable Li-ion batteries are becoming the system of choice because of their overall good performance and high energy density. State-of-the-art Li-ion batteries have an energy density of 125–150 Wh/kg. The energy density of Li-ion batteries can be enhanced by utilizing high energy density cathodes such as, layered lithium manganese oxide, LixMnO2. The theoretical energy density of layered LixMnO2 is ˜950 Wh/kg (based on x=1 and a discharge voltage of 3.3 V), calculated on the basis of the weight of the electrochemically active material. Additionally, LixMnO2 is a low cost and non-toxic material, which makes it very attractive as cathode material for rechargeable Li-ion batteries in commercial, aerospace and military applications. However, LixMnO2 suffers from structural instability during electrochemical cycling and as a result, exhibits significant capacity fade. Furthermore, macrocrystalline materials have poor rate capabilities.
LixMnO2 is found in many structural forms, each of which has a certain stability range so that as x in LixMnO2 is varied, a different phase becomes thermodynamically stable. At low values of x, the spinel structure, LiMn2O4, is stable, whereas at higher x values, a layered structure (e.g., LiMnO2) is stable. At higher values of x (>0.5), the spinel compound transforms to a layered compound with a significant volume change (˜16%) because of the Jahn-Teller distortion; consequently, capacity of cathodes made of the spinel compound fades rapidly on cycling if cathodes are discharged to a higher value of x (>0.5). In case of an unstabilized LixMnO2 layered compound, on charging, a spinel structure is formed; i.e., the manganese ions diffuse from the ordered configuration in the layered structure, and this transformed spinel phase will lose capacity on cycling in a voltage range of 4.2–2V.
In order to provide a stabilized LiMnO2 structure, several researchers have doped this compound with a variety of cation dopants, such as Al3+, Co3+, Ga3+ and Cr3+. One research group prepared monoclinic layered LiMnO2 powders through substitution of Mn in LiMnO2 by small trivalent metal ions. In case of Al and Ga, the layered phase undergoes crystal structural transformations to spinel type phases, along with a capacity fade, during electrochemical cycling. A second research group synthesized LiAl0 25Mn0 75O2 powders. The energy density of cathodes made of these powders in a Li-test cell was 450 Wh/kg and 545 Wh/kg at a discharge rate of C/5 and C/15, respectively. However these energy density numbers are far below the theoretical energy density (˜950 Wh/kg) of layered LiMnO2. The probable reason for these lower capacities of LiAlyMn1−yO2 is the poor rate of diffusion of Li-ions into cathode particles and the poor electronic conductivity of LiMnO2 phase. For instance, the second research group also observed that on raising the operating temperature of Li-test cells from 25° C. to 55° C., the capacity of LiAlyMn1−yO2 increased significantly. A non-nanostructured layered lithium manganese oxide compound of the formula, LiMn1−xAxO2 has been claimed to be produced in U.S. Pat. No. 6,361,756. In this compound, x varies between 0 and 0.5, A is a combination of two or more dopants, and the average oxidation state N of the dopant combination [A] is +2.8≦N≦3.2. Preferably, at least one of the dopants is either Ti or Zr.